1. Field of the Invention
The present invention relates to a method of heat treatment of wire according to which the rolled stock is wound or reeled into coils or the coils are formed with a laying head. The present invention also relates to an installation for effecting the method.
2. Description of the Prior Act
For producing wire having a diameter from 5 mm to 60 mm, usually a long-length rolled product, after being rolled, is wound, with a so-called Garret coiler, or reeled into coils, or separate windings are formed with a laying head, placed on a roller or chain conveyer, and are assembled into a coil at the conveyor end with an aid of a mandrel.
As a result of cooling of the material which takes place in a coiler or a reel, the wire is characterized by different strength along its length due to different cooling conditions within the coil. The different strength, according to known methods, is either compensated by a subsequent treatment of the coil or is eliminated by a subsequent treatment, e.g., quenching and tempering, of the end product.
German Publication DE 28 30 153A1 discloses a method of heat treatment of wire or strip coiled into rings. The rolled stock is coiled, and the formed coil is cooled. Before the heat treatment, the coil or the strip are brought to a predetermined temperature which, as a rule (for steel), corresponds to the austenitization temperature. Austenitized rings are then hardened by excited sympathetic vibrations and, finally, are tempered.
The drawback of the above-described method consists in that the cooling is not uniform, and the wire is cooled noticeably slowly in the center of the coil than at its edge. As a result, thicker oxide layers are formed on the windings located in the center of the coil than on the edge windings. During a subsequent pickling process necessary before further processing of the wire product, also the inner winding need be freed of the oxide layers. This is connected with a danger that the outer windings with low-oxide layers will be attacked by the pickling acid too severely and will be damaged.
European Publication EP 0 849 369A2 discloses a method of heat treatment of wire or steel rods according to which the rolled stock is coiled in a basket or is placed in form of windings, which are formed with a laying head, on a conveyor band and, at the end of the conveyor, are assembled into a coil with a mandril. In order to prevent the variations during cooling in the coiler or during formation of windings and the resulting inhomogeneous mechanical characteristics, it is proposed to already cool the just rolled product, before coiling or formation of the winding, during the travel of the product along the path from the last rolling stand to the cooling region. This cooling is effected in such a way that the outer surface of the rolled product is not undercooled and, thus, reaches into the region of martensite formation. This cooling should prevent hard spots on the surface of the rolled stock. According to the method disclosed in EP 0 849 369 A2, the rolled stock should be converted from the austenite phase into ferrite/perlite phase almost isothermically.
Accordingly, an object of the present invention is a method of heat treatment of wire and an installation for effecting the method which would prevent a non-uniform structure formation and, thereby, would prevent non-uniform mechanical characteristics over the longitudinal extent of the length of the product.
This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a method of heat treatment of wire according to which a steel long-length rolled stock, which has just left the rolling heat region, is cooled to a temperature below a temperature at which martensite formation starts, and then is wound or reeled into a coil, or a coil is formed by using a laying head; and by providing an installation including a rolling mill stand, a device for coiling the rolled stock or laying head means for placing windings on a conveyor and a collection station for forming coils by using a mandrel, which is located downstream of the rolling mill stand, a cooling line located immediately downstream of the rolling mill stand and upstream of the coiling means for cooling the long-length rolled stock leaving the rolling mill stand to a temperature below the temperature of the start of the martensite formation, and a tempering furnace located downstream of the coiling means.
The temperature of the start of the martensite is the temperature at which martensite conversion starts. It is noticeably influenced by an increased carbon content and by alloy additive and is, therefore, depends on special alloy compositions. Only after the rolled stock has been quenched, it is coiled, reeled, or windings are formed. Finally, the tempering step of the quenching and tempering process is effected.
As known, quenching and tempering process, as known, consists of three steps, namely, austenitization of the stock, i.e., of heating-up and heating through of the stock and homogenization of the structure; quenching for obtaining a hard structure; and tempering for improving toughness characteristics. The method according to the present invention eliminates the austenitization step because the wire is cooled immediately after it leaves the rolling heat region. The inventors found out that it was possible to form windings also in the hardened condition and them temper them.